I am interested in buying a used BMW M3 Convertible (probably some
year b/w 1999 and 2004).  I have seen ranging costs for vehicles on
many online sites (Ebay, autotrader, etc) from $23K to $35K.

I live in Wisconsin, and all of the vehicles are being sold out of state.

My question(s) are regarding 

1) Out of state tax
2) WI state tax
3) WI title and registration fee.

1) If I buy this used car in another state, do I have to pay sales tax
to that state?  Does it matter if I buy through a dealer or a private
owner?

2) Do I have to pay WI state sales tax on the car? Again, does it
matter if I bought from a private owner or dealer? Are there any
loopholes / legal ways to get around the tax?

3) What would my title and registration fees be in WI?

If you buy from a private party and drive home to register you pay
your states tax rate. None in the state of purcase.
If you purchase from a dealer out of state you pay THAT states taxes,
and when you come home to register, if your home state tax rate is
higher, you (usually) pay the differential, if its lower you probably
wont have to pay any.
Annual registration in WI is $55
http://dot.wisconsin.gov/drivers/vehicles/veh-fees.htm

Also consider buying in the late fall in a state with harsh winters.
Convertables are usually cheaper than as people are thinking about
warm SUVs or cars.